Oil Condensing Boiler - optimum flow/return temperatures

Qyburn

Hi,

I've seen the comments regarding gas condensing boilers, how they need a suitably low return temperature to operate in condensing mode.  Does the same apply to oil boilers?  We have a Grant Vortex Pro boiler and I've been through both user and the installers' guides and can't see any reference. The boiler has a thermostat for flow temperature, not marked in degrees or even with tick marks.  Any comments or suggestions?

lohr500

Yes, we have a Grant Vortex Blue oil boiler.

Get yourself a couple of these Brannan thermometers if you don't already have a reasonably accurate way to measure the flow and return temperatures.

https://www.amazon.co.uk/Clip-pipe-thermometer-hot-water/dp/B00GYV0L5I/ref=sr_1_12?m=A1UVYP7Z73LFVJ&qid=1672500400&s=merchant-items&sr=1-12

They are available elsewhere including RS.

I run our boiler with a 60C flow and around 40 to 45C return. It depends how cold the rooms are, as the differential is greatest when the rooms are cold.

Adjust the boiler heating thermostat first with a cold system, such that the flow hits 60 and doesn't go higher. It is a case of trial and error. I marked the panel on the boiler with a permanent marker to show the position once I had figured it out.

Then start monitoring the return temperature. You may need to balance the radiators and/or change the circulation pump speed to get the return temperature differential. If the water flows too fast around the heating circuit then the radiators can't shed enough heat into the rooms and the return temperature won't drop low enough. Start with the pump speed if your pump is adjustable.

The boiler will start condensing if the return is less than 54C and the lower the better. But if you run the system with too low a flow/return then the radiators will struggle to heat the rooms.

Depending on your system, you also need to adjust the water storage tank thermostat if you have a hot water tank.
If the tank thermostat is set higher than the boiler flow temperature (controlled by the boiler thermostat on the control panel photo), then the boiler will cycle on and off frequently when you have the hot water setting enabled. 

I have our water tank thermostat set to 50C which gives us enough hot water for our needs. There is advice that the hot water should be boosted to 60C once a week to kill off any legionella bacteria, but I have never done this as we have a reasonable flow of water through our hot water tank.

Hope this helps. I am no expert, but the above seemed to work for me and once set, other than occasionally monitoring the pipe thermometers, the system can be left alone.

Qyburn

Thanks. There's lots of information relating to gas boilers, but not much referring to oil. Since the composition of the flue gases is very different I wasn't sure that the same temperature ranges would apply. In particular there is less water in oil exhaust.

I found an earlier post on another board, referring to an earlier model Grant oil boiler. At that time Grant said to keep the return temperature above 40 deg to avoid harmful corrosion. I think I'll ask them for specifics about my exact model

Thanks for the thermometer recommendation, I will look out a suitable version to suit our pipework.

lohr500

Good point. It looks like 40C min is still the recommendation for the latest Vortex Pro :

To achieve the maximum efficiency possible from the Grant Vortex boiler, the heating system should be designed to the following parameters: RADIATORS: • Flow temperature 70°C • Return temperature 50°C • Differential 20°C Size radiators with a mean water temperature of 60°C. Design system controls with programmable room thermostats or use weather compensating controls to maintain return temperatures below 55°C. ! NOTE ! The boiler should not be allowed to operate with return temperatures of less than 40°C when the system is up to temperature. The use of a pipe thermostat is recommended to control the return temperature when using weather compensating controls. UNDERFLOOR: • Flow temperature 50°C • Return temperature 40°C • Differential 10°C In underfloor systems, it is essential that the return temperature must be maintained at or above 40°C to prevent internal corrosion of the boiler water jacket.

Taken from the latest installer manual on the Grant website

https://www.grantuk.com/media/5033/grant-vortex-pro-and-pro-system-installer-uk-doc-0120-rev-3-0-july-2022.pdf

Based on this, I am still OK with the return on mine being between 40C and 45C.

Qyburn

Cheers, that's great info. Pipe thermometers are on order. Our radiators are old and probably undersized compared to boiler output so I don't want to drop too low, I'll probably aim for 50 return temp.

lohr500

This graph, taken from the Urban Plumbers website shows efficiency vs return temperature



There is around a 4% improvement running at 40 vs 50, so worth trying to get closer to a 40C return if you can keep the house warm with your existing radiators.
But yes, why not start at 50 and then try reducing a little at a time to see if the rooms stay warm enough.